Method and apparatus for removing contamination

ABSTRACT

Cleaning equipment for an EUV wafer chuck or clamp, which removes particles that have accumulated between burls on the surface of the wafer chuck. The equipment includes a spinning bi-polar electrode placed in proximity to the surface, which can attract and adsorb the charged particle residue therefrom using its generated symmetric electric field when the wafer chuck is not in use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 17/487,006 filed on Sep. 28, 2021, which claims priority under35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 63/166,895entitled “EUV WAFER TABLE CLEANING DEVICE” filed on Mar. 26, 2021, theentirety of which is hereby incorporated by reference.

BACKGROUND

In semiconductor device manufacturing, various types of plasma processesare used to deposit layers of conductive and dielectric material onsemiconductor substrates, and also to blanket etch and selectively etchmaterials from the substrate. One growing technique for semiconductormanufacturing is extreme ultraviolet (EUV) lithography, which employsscanners using light in the EUV spectrum of electromagnetic radiation,including wavelengths from about one nanometer (nm) to about 100 nm.During such processes, the substrate is affixed to a substrate chuck ina process chamber and a plasma is generated adjacent the substratesurface. Various techniques have evolved to affix the substrate to thesubstrate chuck. For example, an electrostatic chuck (ESC) can be usedto hold the substrate during the plasma processes. The use of anelectrostatic chuck eliminates the need for mechanical clamp rings, andgreatly reduces the probability of forming particles by abrasion etc.,which particles cause yield problems and require frequent cleaning ofthe apparatus.

Even though the use of an electrostatic chuck reduces particlecontamination, it is inevitable that small debris particles arenonetheless formed over time, and other contamination is generatedwithin the process chamber during normal operation and/or cleaning.These particles and contamination when deposited or formed on thesubstrate chuck surface of an ESC increase the leakage of the heattransfer gas at the interface of the chuck surface and substrate. Thisleakage reduces the temperature control of the substrate and theefficiency of substrate cooling techniques. Consequently, the processchamber and the substrate chuck must be cleaned more frequently. Thisresults in down-time for the apparatus, and requires an expensive andtime consuming manual apparatus cleaning operation. Therefore, there isa need for improving the cleaning process without substantial down-time.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1A is a diagram of a lithography apparatus in accordance with someembodiments.

FIG. 1B is a diagram of a source side and a scanner side in accordancewith some embodiments.

FIG. 2A is a diagram of a wafer stage in accordance with someembodiments.

FIG. 2B is a diagram of a stone cleaning process in accordance with someembodiments.

FIG. 3A is a first diagram of a spinning electrode and a wafer clamp inaccordance with some embodiments.

FIG. 3B is a second diagram of a spinning electrode and a wafer clamp inaccordance with some embodiments.

FIG. 3C is a diagram of alternate designs of the spinning electrode inaccordance with some embodiments.

FIG. 4A is a diagram of measured contamination of the wafer clamp beforestone cleaning in accordance with some embodiments.

FIG. 4B is a diagram of measured contamination of the wafer clamp afterstone cleaning in accordance with some embodiments.

FIG. 4C is a diagram of measured contamination captured by the spinningelectrode in accordance with some embodiments.

FIG. 5A and FIG. 5B are diagrams of a controller in accordance with someembodiments.

FIG. 6 is a flowchart of a cleaning process using the spinning electrodein accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and also include embodiments in whichadditional features are formed between the first and second features,such that the first and second features are not in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, are used herein for ease of descriptionto describe one element or feature's relationship to another element(s)or feature(s) as illustrated in the figures. The spatially relativeterms are intended to encompass different orientations of the device inuse or operation in addition to the orientation depicted in the figures.The apparatus/device may be otherwise oriented (rotated 90 degrees or atother orientations) and the spatially relative descriptors used hereinmay likewise be interpreted accordingly. In addition, the term “made of”may mean either “comprising” or “consisting of.” In the presentdisclosure, a phrase “one of A, B and C” means “A, B and/or C” (A, B, C,A and B, A and C, B and C, or A, B and C), and does not mean one elementfrom A, one element from B and one element from C, unless otherwisedescribed.

As used herein, the term “optic” is meant to be broadly construed toinclude, and not necessarily be limited to, one or more components whichreflect and/or transmit and/or operate on incident light, and includes,but is not limited to, one or more lenses, windows, filters, wedges,prisms, grisms, gratings, transmission fibers, etalons, diffusers,homogenizers, detectors and other instrument components, apertures,axicons and mirrors including multi-layer mirrors, near-normal incidencemirrors, grazing incidence mirrors, specular reflectors, diffusereflectors and combinations thereof. Moreover, unless otherwisespecified, the term “optic,” as used herein, is not meant to be limitedto components which operate solely within one or more specificwavelength range(s) such as at the EUV output light wavelength, theirradiation laser wavelength, a wavelength suitable for metrology or anyother specific wavelength.

In the present disclosure, the terms “mask,” “photomask,” and “reticle”are used interchangeably. In the present embodiment, the mask is areflective mask. One embodiment of the mask includes a substrate with asuitable material, such as a low thermal expansion material or fusedquartz. In various examples, the material includes TiO₂ doped SiO₂, orother suitable materials with low thermal expansion. The mask includesmultiple reflective layers deposited on the substrate. The multiplelayers include a plurality of film pairs, such as molybdenum-silicon(Mo/Si) film pairs (e.g., a layer of molybdenum above or below a layerof silicon in each film pair). Alternatively, the multiple layers mayinclude molybdenum-beryllium (Mo/Be) film pairs, or other suitablematerials that are configurable to highly reflect the EUV light. Themask may further include a capping layer, such as ruthenium (Ru),disposed on the ML for protection. The mask further includes anabsorption layer, such as a tantalum boron nitride (TaBN) layer,deposited over the multiple layers. The absorption layer is patterned todefine a layer of an integrated circuit (IC). Alternatively, anotherreflective layer may be deposited over the multiple layers and ispatterned to define a layer of an integrated circuit, thereby forming anEUV phase shift mask.

In the present embodiments, the semiconductor substrate is asemiconductor wafer, such as a silicon wafer or other type of wafer tobe patterned. The semiconductor substrate is coated with a resist layersensitive to the EUV light in the present embodiment. Various componentsincluding those described above are integrated together and are operableto perform various lithography exposing processes. The lithographysystem may further include other modules or be integrated with (or becoupled with) other modules.

A lithography system is essentially a light projection system. Light isprojected through a ‘mask’ or ‘reticle’ that constitutes a blueprint ofthe pattern that will be printed on a workpiece. The blueprint is fourtimes larger than the intended pattern on the wafer or chip. With thepattern encoded in the light, the system's optics shrink and focus thepattern onto a photosensitive silicon wafer. After the pattern isprinted, the system moves the wafer slightly and makes another copy onthe wafer. This process is repeated until the wafer is covered inpatterns, completing one layer of the eventual semiconductor device. Tomake an entire microchip, this process will be repeated one hundredtimes or more, laying patterns on top of patterns. The size of thefeatures to be printed varies depending on the layer, which means thatdifferent types of lithography systems are used for different layers,from the latest-generation EUV systems for the smallest features toolder deep ultraviolet (DUV) systems for the largest.

FIG. 1A is a schematic and diagrammatic view of an EUV lithographysystem 10. The EUV lithography system 10 includes an EUV radiationsource apparatus 100 (sometimes referred to herein as a “source side” inreference to it or one or more of its relevant parts) to generate EUVlight, an exposure tool 300, such as a scanner, and an excitation lasersource apparatus 200. As shown in FIG. 1A, in some embodiments, the EUVradiation source apparatus 100 and the exposure tool 300 are installedon a main floor (MF) of a clean room, while the excitation laser sourceapparatus 200 is installed in a base floor (BF) located under the mainfloor. Each of the EUV radiation source apparatus 100 and the exposuretool 300 are placed over pedestal plates PP1 and PP2 via dampers DP1 andDP2, respectively. The EUV radiation source apparatus 100 and theexposure tool 300 are coupled to each other at a junction 330 by acoupling mechanism, which may include a focusing unit (not shown).

The EUV lithography system 10 is designed to expose a resist layer toEUV light (or EUV radiation). The resist layer is a material sensitiveto the EUV light. The EUV lithography system 10 employs the EUVradiation source apparatus 100 to generate EUV light having a wavelengthranging between about 1 nanometer (nm) and about 100 nm. In oneembodiment, the EUV radiation source apparatus 100 generates EUV lightwith a wavelength centered at about 13.5 nm. In various embodiments, theEUV radiation source apparatus 100 utilizes laser produced plasma (LPP)to generate the EUV radiation.

As shown in FIG. 1A, the EUV radiation source apparatus 100 includes atarget droplet generator 115 and an LPP collector 110, enclosed by achamber 105. The target droplet generator 115 generates a plurality oftarget droplets 116. In some embodiments, the target droplets 116 aretin (Sn) droplets. In some embodiments, the target droplets 116 have adiameter of about 30 microns (μm). In some embodiments, the targetdroplets 116 are generated at a rate of about fifty droplets per secondand are introduced into an excitation zone 106 at a speed of aboutseventy meters per second (m/s or mps). Other material can also be usedfor the target droplets 116, for example, a liquid material such as aeutectic alloy containing Sn and lithium (Li).

As the target droplets 116 move through the excitation zone 106,pre-pulses (not shown) of the laser light first heat the target droplets116 and transform them into lower-density target plumes. Then, the mainpulse 232 of laser light is directed through windows or lenses (notshown) into the excitation zone 106 to transform the target plumes intoan LPP. The windows or lenses are composed of a suitable materialsubstantially transparent to the pre-pulses and the main pulse 232 ofthe laser. The generation of the pre-pulses and the main pulse 232 issynchronized with the generation of the target droplets 116. In variousembodiments, the pre-heat laser pulses have a spot size about 100 μm orless, and the main laser pulses have a spot size about 200-300 μm. Adelay between the pre-pulse and the main pulse 232 is controlled toallow the target plume to form and to expand to an optimal size andgeometry. When the main pulse 232 heats the target plume, ahigh-temperature LPP is generated. The LPP emits EUV radiation, which iscollected by one or more mirrors of the LPP collector 110. Moreparticularly, the LPP collector 110 has a reflection surface thatreflects and focuses the EUV radiation for the lithography exposingprocesses. In some embodiments, a droplet catcher 120 is installedopposite the target droplet generator 115. The droplet catcher 120 isused for catching excess target droplets 116 for example, when one ormore target droplets 116 are purposely or otherwise missed by thepre-pulses or main pulse 232.

As shown, the target droplet generator 115 generates tin droplets alonga vertical axis. Each droplet is hit by a CO₂ laser pre-pulse (PP). Thedroplet will responsively change its shape into a “pancake” during itstravel along the axial direction. After a time duration (PP to MP delaytime), the pancake is hit by a CO₂ laser main pulse (MP) proximate to aprimary focus (PF) in order to generate an EUV light pulse. The EUVlight pulse is then collected by an LPP collector 100 and delivered tothe exposure tool 300 for use in wafer exposure.

The LPP collector 110 includes a proper coating material and shape tofunction as a mirror for EUV collection, reflection, and focusing. Insome embodiments, the LPP collector 110 is designed to have anellipsoidal geometry. In some embodiments, the coating material of thecollector 100 is similar to the reflective multilayer of an EUV mask. Insome examples, the coating material of the LPP collector 110 includesmultiple layers, such as a plurality of molybdenum/silicon (Mo/Si) filmpairs, and may further include a capping layer (such as ruthenium (Ru))coated on the multiple layers to substantially reflect the EUV light.

The main pulse 232 is generated by the excitation laser source apparatus200. In some embodiments, the excitation laser source apparatus 200includes a pre-heat laser and a main laser. The pre-heat laser generatesthe pre-pulse that is used to heat or pre-heat the target droplet 116 inorder to create a low-density target plume, which is subsequently heated(or reheated) by the main pulse 232, thereby generating increasedemission of EUV light.

The excitation laser source apparatus 200 may include a laser generator210, laser guide optics 220 and a focusing apparatus 230. In someembodiments, the laser generator 210 includes a carbon dioxide (CO₂)laser source or a neodymium-doped yttrium aluminum garnet (Nd:YAG) lasersource. The laser light 231 generated by the laser generator 210 isguided by the laser guide optics 220 and focused into the main pulse 232of the excitation laser by the focusing apparatus 230, and thenintroduced into the EUV radiation source apparatus 100 through one ormore apertures, such as the aforementioned windows or lenses.

In such an EUV radiation source apparatus 100, the LPP generated by themain pulse 232 creates physical debris, such as ions, gases and atoms ofthe droplet 116, along with the desired EUV light. In operation of thelithography system 10, there is an accumulation of such debris on theLPP collector 110, and such physical debris exits the chamber 105 andenters the exposure tool 300 (i.e., the “scanner side”) as well as theexcitation laser source apparatus 200.

In various embodiments, a buffer gas is supplied from a first buffer gassupply 130 through the aperture in the LPP collector 110 by which themain pulse 232 of laser light is delivered to the tin droplets 116. Insome embodiments, the buffer gas is hydrogen (H₂), helium (He), argon(Ar), nitrogen (N₂), or another inert gas. In certain embodiments, H₂ isused, since H radicals generated by ionization of the buffer gas canalso be used for cleaning purposes. Furthermore, H₂ absorbs the leastamount of EUV light produced by the source side, and thus absorbs theleast light used by the semiconductor manufacturing operations performedin the scanner side of the lithography apparatus 10. The buffer gas canalso be provided through one or more second buffer gas supplies 135toward the LPP collector 110 and/or around the edges of the LPPcollector 110. Further, and as described in more detail later below, thechamber 105 includes one or more gas outlets 140 so that the buffer gasis exhausted outside the chamber 105.

Hydrogen gas has low absorption of the EUV radiation. Hydrogen gasreaching to the coating surface of the LPP collector 110 reactschemically with a metal of the target droplet 116, thus forming ahydride, e.g., metal hydride. When Sn is used as the target droplet 116,stannane (SnH₄), which is a gaseous byproduct of the EUV generationprocess, is formed. The gaseous SnH₄ is then pumped out through theoutlet 140. However, it is difficult to exhaust all gaseous SnH₄ fromthe chamber and to prevent the Sn debris and SnH₄ from entering theexposure tool 300 and the excitation laser source apparatus 200. To trapthe Sn, SnH₄ or other debris, one or more debris collection mechanismsor devices 150 are employed in the chamber 105. In various embodiments,a controller 500 controls the EUV lithography system 10 and/or one ormore of its components shown in and described above with respect to FIG.1A.

As shown in FIG. 1B, the exposure tool 300 (sometimes referred to hereinas the “scanner side” in reference to it or one or more of its relevantparts) includes various reflective optic components, such asconvex/concave/flat mirrors, a mask holding mechanism 310 including amask stage (i.e., a reticle stage), and wafer holding mechanism 320. TheEUV radiation generated by the EUV radiation source apparatus 100 andfocused at the intermediate focus 160 is guided by the reflectiveoptical components 305 onto a mask (not shown) secured on the reticlestage 310, also referenced as a “mask stage” herein, within theprocessing chamber 306. In various embodiments of the EUV lithographysystem 10, pressure in the LPP source side is higher than pressure inthe scanner side. This is because the source side uses hydrogen gas toforce the removal of airborne Sn debris therefrom, while the scannerside is maintained in near vacuum in order to avoid diminishing thestrength of the EUV light (being absorbed by air molecules) or otherwiseinterfering with the semiconductor manufacturing operations performedtherein. In various embodiments, the intermediate focus 160 is disposedat the junction 330, namely, the intersection of the source side and thescanner side.

In some embodiments, the distance from the intermediate focus 160 andthe reticle disposed in the scanner side is approximately 2 meters. Insome embodiments, the reticle size is approximately 152 mm by 152 mm. Insome embodiments, the reticle stage 310 includes an electrostatic chuck,or ‘e-chuck,’ to secure the mask. The EUV light patterned by the mask isused to process a wafer supported on wafer stage 320. Because gasmolecules absorb EUV light, the chambers and areas of the lithographysystem 10 used for EUV lithography patterning are maintained in a vacuumor a low-pressure environment to avoid EUV intensity loss. Nonetheless,a debris collector 370, similar in purpose and design to the debriscollector 150, may be provided in the scanner side to dispose of andremove contamination and residue that may accumulate in the componentsof the scanner side. In various embodiments, the controller 500 controlsone or more of the components of the EUV lithography system 10 as shownin and described with respect to FIG. 1B.

FIG. 2A is a diagram of a wafer stage 320 in accordance with someembodiments. The wafer stage 320 includes a wafer clamp 322, alsoreferred to herein as a wafer chuck. In various embodiments, the waferchuck 322 is an electrostatic chuck (ESC) disposed within the processchamber 306, and the ESC is configured to receive a substrate 326. Inaccordance with various embodiments, the substrate 326 includes a wafer,silicon substrate, or any other wafer, workpiece or substrate. Invarious embodiments, the apparatus 10 also includes an internal chuckelectrode (not shown), and a source of direct current (DC) powerconnected to the chuck electrode in order to provide power thereto.

In various embodiments, the substrate 326 is clamped by theelectrostatic chuck 322 by an electrostatic potential. In variousembodiments, when a DC voltage from the source of DC power (not shown)is applied to the chuck electrode of the electrostatic chuck 322 havingthe substrate 326 disposed thereon, a Coulomb force is generated betweenthe substrate 326 and the chuck electrode. The Coulomb force attractsand holds the substrate 326 on the electrostatic chuck 322 until theapplication of the DC voltage from the source of DC power isdiscontinued. In various embodiments, the applied DC voltage ranges fromabout 2000 volts to about 3200 volts. In some embodiments, the appliedDC voltage is 3000 volts. In some embodiments, the source of DC power isconfigured to apply the DC voltage of about 10% to about 90% of thepower applied during normal etching operations using direct current. Insome embodiments, the source of DC power is configured to apply the DCvoltage of about 20% to about 80%, about 30% to about 70%, or about 30%to about 50% of the power applied during normal lithography operationsusing direct current. In some embodiments, the application of the DCvoltage occurs for a duration of about 10 seconds to about 60 seconds,about 10 seconds to about 50 seconds, about 20 seconds to about 40seconds, or about 20 seconds to about 30 seconds.

In various embodiments, the surface of the electrostatic chuck 322includes a plurality of burls 324, which have a width in a range fromabout 100 μm to about 500 μm, a height from the surface of theelectrostatic chuck 322 in a range from about 1.0 μm to about 100 μm,and are spaced from each other in a range from about 1.0 μm to about 5.0μm. In various embodiments, the electrostatic chuck 322 has greater thantwo thousand such burls 324, comprising roughly 1.5% of the surface areaof the surface of the electrostatic chuck 322, in order to support thesubstrate 326.

In various embodiments, the application of the DC power positivelycharges particles or contaminants on the surface of the substrate 326.In various embodiments, the application of the DC power negativelycharges particles or contaminants on the surface of the wafer clamp 322,which may settle upon the tops of the burls 324. In some embodiments,debris particles or contaminants are generated from the substrate 326during lithography operations and/or due to the electrostatic action ofthe wafer clamp 322. The debris particles include, but are not limitedto the following elements: Cu, Al, Ni, Ti, O, F, Si, Cu, Al, Ge, Ni, Ti,W, Xi, Mo, Fe, Pb, Bi, and In, or alloys or compounds thereof. In someembodiments, the debris particles include molecules such as:Si_(x)O_(y), Al_(x)O_(y), and Ti_(x)O_(y). The surface debris particlesand/or contaminants must be removed from the electrostatic chuck 330 inorder to maximize the manufacturing performance of the apparatus 10.

FIG. 2B is a diagram of a stone cleaning process in accordance with someembodiments. The purpose of the stone cleaning is to remove the largedebris particles 340 that accumulate on top of the burls 324 of theelectrostatic chuck 322 over time and with usage. In variousembodiments, the stone cleaning system automatically cleans excessivefocus spots caused by debris particles 340 on the burls 324 of theelectrostatic chuck 322 without operator intervention. In variousembodiments, the stone cleaning system uses a cleaning stone 328 in aholder (not shown) that grinds the burls 324 of the electrostatic chuck322 as it is moved by a wafer chuck mover 380 underneath the stone 328.In some embodiments, the stone is composed of marble. In variousembodiments, the wafer clamp 322 is moved laterally (i.e., back andforth horizontally and/or vertically) and/or circularly (i.e., inexpanding concentric circles or spirals) with respect to the stone 328during this cleaning operation until the entire surface area of thewafer clamp 322 has been treated. In some embodiments, the stonecleaning process is manually performed by an operator.

In various embodiments, the apparatus 10 also includes a spectral and/orcharge monitoring system 385. The spectral and/or charge monitoringsystem 385 is configured to monitor surface charge level and/orcomposition of debris particles and/or contaminants 340, 342 on thesurface of the electrostatic chuck 322. In some embodiments, thespectral and/or charge monitoring system 385 uses x-rays or an ion beamto charge the debris particles and/or contaminants 340, 342 includingany by-products. For example, during a stone cleaning operation of theapparatus 10, the spectral and/or charge monitoring system 385 isconfigured to continuously or periodically monitor surface charge leveland/or composition of debris particles and/or contaminants 340, 342. Insome embodiments, continuous or periodic monitoring by the spectraland/or charge monitoring system 385 automatically determines whencleaning is needed and/or needs to be continued. In some embodiments, itprovides a user with a contamination history or profile of the apparatus10 throughout its service life, or any time period thereof.

Through use of the spectral and/or charge monitoring system 385, it hasbeen discovered that the stone cleaning process can only clean the largeparticles 340 that exist on the top of burls 324, and generatesadditional, smaller, electrically-charged particles 342 that settlebetween the burls 324 after stone cleaning in some embodiments. Theseresidue particles 342 build up over time with machine usage andinterfere with the efficiency and alignment of the electrostatic chuck322, particularly impacting wafer exposure quality, such as overlay.

It has been found that the smaller residue particles 342 that settlebetween the burls 324 are not affected by further applications of thestone cleaning process, yet the particles 342 should nonetheless beremoved. Accordingly, a spinning bi-polar electrode disk is introducedto sweep across the wafer chuck 322 (while it is inactive) in order toabsorb the charged particles 342 that have settled between the burls 324after stone cleaning in some embodiments.

FIG. 3A is a first diagram of a spinning cleaning electrode 360 providedabove the wafer clamp 322 in accordance with some embodiments. Invarious embodiments, the wafer chuck mover 380 moves the wafer clamp 322with respect to the spinning cleaning electrode 360. In variousembodiments, the wafer clamp 322 is moved laterally (i.e., back andforth and/or left and right horizontally and/or vertically) and/orcircularly (i.e., in expanding concentric circles) with respect to thebi-polar cleaning electrode 360 while the electrode is spinning duringthis additional cleaning operation until the entire surface area of thewafer clamp 322 has been treated. In other embodiments, the bi-polarcleaning electrode 360 moves over the wafer clamp 322 while the waferclamp 322 is stationary, or both the bi-polar cleaning electrode 360 andthe wafer clamp 322 are moved simultaneously.

The bi-polar cleaning electrode 360 is designed for electric adsorptionin the near-vacuum environment of the EUV lithography apparatus 10. Invarious embodiments, the bi-polar cleaning electrode 360 sweeps alongthe surface of the wafer clamp 322 just above the burls 324, withoutphysical contact. The bi-polar electrode then attracts and adsorbs bothpositively-and negatively-charged debris particles 342 from between theburls 342 of the wafer clamp 322 using a symmetric rotating bi-polarelectric field in various embodiments.

A symmetric electric field yields optimal results for particleattraction and removal of debris particles. This is because anasymmetric electric field jostles particles non-uniformly, namely invarious directions including directions away from the surface of thecleaning electrode 360. This, in turn, causes inadvertent andundesirable spreading of the debris particles from the wafer chucksurface into the processing chamber 306.

In order to generate a symmetric electric field using a shapedelectrode, it is necessary to use an electrode shape that issubstantially symmetrical with at least two axes of symmetry. Shapesthat have a single axis of symmetry will not generate symmetric electricfields in all directions. The greater the number of axes of symmetry,the more symmetric the electric field that will be produced. A circlehas infinite axes of two-dimensional symmetry. Accordingly, as shown inFIG. 3A, the shape of the cleaning electrode 360 is circular in variousembodiments.

In order to generate an electric field, the surface of the cleaningelectrode 360 is charged from a power source. In order to be charged,the cleaning electrode 360 is electrically conductive. In order to avoidinterference with other components of the apparatus 10, the cleaningelectrode 360 is not be magnetic or radioactive in various embodiments.Accordingly, in various embodiments, the cleaning electrode 360 is madeof one or more of the following conductive elements (or moleculescontaining the same) in solid form: silver, copper, gold, aluminum,calcium, beryllium, rhodium, magnesium, molybdenum, tungsten, zinc,cobalt, cadmium, nickel, lithium, iron, platinum, palladium, tin,selenium, tantalum, niobium, chromium, lead, vanadium, antimony,zirconium and titanium. In some embodiments, the cleaning electrode 360is composed of cast steel or stainless steel.

Applying electric power to a conductive electrode shape will notnecessarily generate a bi-polar symmetric field as needed for thepresent processes using a cleaning electrode. For example, simplyapplying opposing power leads to opposing surfaces of a circularconductive shape will yield a capacitor-like element where the topsurface of the shape is, for example, positively charged, and the bottomsurface of the shape is accordingly negatively charged. Such aconfiguration would not be optimal for cleaning electrically-chargesparticles 342 from the surface of the wafer chuck 322, since the topsurface would only attract negatively-charged particles while repellingthe positively charged particles, and the bottom surface would onlyattract positively-charged particles while repelling and dislodgingnegatively-charged particles. In the present embodiments, bothpositively-charged and negatively-charged particles are distributed asdebris particles 324. Accordingly, for desirable performance inattracting both types of charged particles, the cleaning electrode 360generates a symmetrical bi-polar electric field where a first pole isformed at one end of the surface and a second, oppositely charged poleis formed at an opposite end of the same surface from the first pole. Invarious embodiments, the cleaning electrode 360 has a diameter of 30±5cm.

In order to accomplish this, a first portion of the surface where thefirst pole resides is electrically isolated from a second portion of thesurface where the second pole resides. This is accomplished in variousembodiments by placing an insulator material 363 between the first andsecond portions. Examples of good insulating material 363 include butare not limited to: poly-vinyl chloride (PVC), glass, rigid laminates,plastic resin, polytetrafluoroethylene, an air gap and rubber. Thepositive lead of an electrical power supply is attached to that portionwhere the positive pole is desired and the negative lead of theelectrical power supply is attached to the remaining portion. In variousembodiments, the poles are disposed on extreme opposite endssubstantially at the center of the electrode surface. Upon theapplication of power, such as DC power, the cleaning electrode 360 willgenerate a bi-polar electric field with the poles formed at the desiredlocations.

Accordingly, with reference to FIG. 3A, the cleaning electrode 360 has acircular shape that includes a first semicircle 362 and a secondsemicircle 364, in various embodiments. In such embodiments, the firstsemicircle 362 is electrically insulated from the second semicircle 364by insulating material 363. In some embodiments, insulating material 363is disposed entirely around a periphery of each semicircle 362, 364. Insome embodiments, the first semicircle 362 is positively-charged by apositive lead of the power source and the second semicircle 364 isnegatively-charged by a negative lead of the power source. In suchembodiments, the first semicircle 362 has the positive pole and thesecond semicircle 364 has the negative pole of the bi-polar symmetricelectric field generated by the circular shape of the cleaning electrode360 when it is charged. In various embodiments, the positive pole andnegative pole are charged to the same absolute value so as to maintainsymmetry of the bi-polar electric field generated thereby. In variousembodiments, the first semicircle 362 and the second semicircle 364 areequally sized and each includes approximately one-half of the shape ofthe cleaning electrode 360. In other embodiments, the first semicircle362 and second semicircle 364 are equal in size but less than one-halfof the total surface area of the cleaning electrode 360, such as ⅓, ¼, ⅛or the like, and the remaining portion is insulating material. Thecharged, bi-polar symmetric cleaning electrode 360 is swept across thesurface of the wafer clamp 322 in a directional manner as shown, invarious embodiments, to dislodge and attract particle debris 342 frombetween the burls 324 thereof. The wafer clamp mover 390 moves the waferclamp 322 directionally (i.e., laterally and/or circularly) with respectto the cleaning electrode 360 in various embodiments. In variousembodiments, the cleaning electrode 360 is placed 0.1 mm to 1 cm abovethe wafer clamp surface depending on the strength of the electric field(e.g., the weaker the field, the closer the cleaning electrode 360 ispositioned). In various embodiments, the spectral and/or chargemonitoring system 385 monitors the contaminant level of the surface ofthe wafer clamp 322, and the data therefrom is used by a controller 500or the like to automatically continue or discontinue cleaning of thewafer clamp 322.

FIG. 3B is a second diagram of the bi-polar cleaning electrode 360 inaccordance with some embodiments. Simply moving a static chargedsymmetrical bi-polar cleaning electrode 360 will not optimally removethe particle debris 342 from the surface of the wafer clamp 322. This isbecause negatively charged particles will be jostled and repelledinstead of attracted by the negative semicircle 364 of the cleaningelectrode 360, and positively charged particles 342 will likewise bejostled and repelled instead of attracted by the positive semicircle 362of the cleaning electrode 360. Accordingly, it has been found thatrotating or spinning the cleaning electrode 360 as it sweeps across theface of the wafer clamp 322 yields desirable particle removal results.

In order to spin the cleaning electrode 360, a motor 390, such as a DCmotor or an alternating current (AC) motor, is attached to the cleaningelectrode 360 via an axle 391 or the like. The motor 390 may be poweredfrom the same or a separate AC or DC power source that is used to chargethe cleaning electrode 360. The motor 390, when powered, rotates theaxle 391, which in turn rotates the cleaning electrode 360. The motor390 may be rated at between one and one hundred volts, in variousembodiments. In various embodiments, the motor 390 spins the electrodeat between 10 and 1000 rotations per minute (RPM). In variousembodiments, the cleaning electrode 360 is spun in either clockwise orcounter-clockwise directions. In some embodiments, the cleaningelectrode 360 may be spun in a clockwise direction for a period,followed by being spun in the opposite direction for a period during thecleaning cycle.

In order to charge the cleaning electrode 360 while it is spun by themotor 390, leads are provided through the axle 391 such that they do notget spun up and tangled during its operation. Accordingly, the leads tocharge the cleaning electrode 360 may include a power transfer device392 such as an internal hard-wired slip ring, or a wireless powerdevice.

A slip ring is an electromechanical device that allows the transmissionof power and electrical signals from a stationary to a rotatingstructure, such as the motor 390 and the cleaning electrode 360. Theslip ring improves system operation by eliminating the need fordamage-prone wires that dangle from movable joints. Also called rotaryelectrical interfaces, rotating electrical connectors, collectors,swivels, or electrical rotary joints. A slip ring includes a stationarygraphite or metal contact (brush) which rubs on the outside diameter ofa rotating metal ring. As the metal ring turns, the electric current orsignal is conducted through the stationary brush to the metal ringmaking the connection. Additional ring/brush assemblies are stackedalong the rotating axis if more than one electrical circuit is needed.Either the brushes or the rings are stationary while the other componentrotates.

In other embodiments, wireless power transmitters and receivers are usedto provide power to charge the cleaning electrode 360 in place of sliprings. Wireless power transmitters include devices, such as inductivepower transmitters that transmit power from an AC or DC power source toinductive power receivers (not shown) on or within the cleaningelectrode 360. Other methods of powering the spinning cleaning electrode360 are readily contemplated.

As shown in FIG. 3B, the spinning cleaning electrode 360 is swept overthe entire surface area of the wafer clamp 322 in various embodiments.Positively and negatively charged particle 342 are attracted by thebi-polar symmetric electric field generated by the spinning cleaningelectrode 360 and adsorbed (not absorbed) onto the surface of thespinning cleaning electrode 360. The debris particles 342 are held inplace on the surface of the spinning cleaning electrode 360 by coulombforces generated by a sufficiently strong electric field in variousembodiments. The process of cleaning the surface of the wafer clamp 322in this manner takes 1-5 complete passes of the cleaning electrode 360over the entire surface area of the wafer clamp 322 in some embodiments.In various embodiments, the cleaning process using the cleaningelectrode 360 is programmed to run for 10±2 minutes. In variousembodiments, such cleaning processes are initiated periodically or aftera threshold number of uses of the wafer clamp 322 during manufacturingby the apparatus 10. The spectral and/or charge monitoring system 385may monitor the contaminant level of the surface of the wafer clamp 322and the data therefrom may be used by a controller or the like toautomatically continue or discontinue cleaning of the wafer clamp 322based on the residual particle debris remaining. Adsorbed debrisparticles 342 are shown attached to the bottom surface of the cleaningelectrode 360 after being attracted off the surface of the wafer chuck322 using the generated symmetric electric field.

FIG. 3C is a diagram of alternate designs of the symmetrical cleaningelectrode 360 in accordance with some embodiments. In some embodiments,the cleaning electrode 360 is not circular, but is instead formed as aregular polygon, where all the sides of the polygon are equal, and allthe interior angles are the same. Diagram 365 shows types of availableregular simple polygons including squares, pentagons, hexagons,heptagons, and so forth as the number of corners of the polygon areincreased. Any regular n-polygon can be used and the electric fieldgenerated thereby will be increasingly symmetrical as the numbers ofcorners are increased, since the number of symmetrical axes increase inproportion to the number of corners. In addition, the surface areaavailable to adsorb debris particles 342 also increases with theincreasing number of corners.

In some embodiments, complex polygons can be used in place of a circularshape for the cleaning electrode 360, since they demonstrate symmetrywith two or more symmetrical axes. Complex polygons, also calledself-intersecting polygons, have sides that cross over each other. Theclassic star is a complex polygon. Such a “regular star polygon” is aself-intersecting, equilateral equiangular polygon. A regular starpolygon is denoted by its Schläfli symbol {p/q}, where p (the number ofvertices) and q (the density) are relatively prime (they share nofactors) and q≥2. The density of a polygon can also be called itsturning number, the sum of the turn angles of all the vertices dividedby 360°. Diagram 366 shows various complex polygon shapes that may beused to form the cleaning electrode 360 in various embodiments. Complexpolygons will be increasingly symmetrical as the numbers of points areincreased, since the number of symmetrical axes increases in proportionto the number of points. In addition, the surface area available toadsorb debris particles 342 also increases with the increasing number ofpoints, and with increasing thickness of the same.

In some embodiments, fan shapes can be used in place of a circular shapefor the cleaning electrode 360, since they also demonstrate symmetrywith two or more symmetrical axes. Diagram 367 illustrates various fanshapes that may be used as the cleaning electrode 360 in place of acircular shape. Such fan shapes will be increasingly symmetrical as thenumbers of “blades” are increased, since the number of symmetrical axesincrease in proportion to the number of such blades. In addition, thesurface area available to adsorb debris particles 342 also increaseswith the increasing number and thickness of the blades. In these variousembodiments, the simple polygon, complex polygon and fan-shapedelectrodes described above will still need to have electrically isolatedfirst and second portions, as described with respect to circular shapes,in order to generate a symmetrical bi-polar electric field.

FIG. 4A is a diagram 402 of measured contamination of the wafer clampbefore stone cleaning in accordance with some embodiments, as measuredby the spectral and/or charge monitoring system 385 or the like invarious embodiments. As shown therein, a large contaminant debrisparticle 340 has formed on an area of the surface of the wafer clamp 322before stone cleaning is applied. In various embodiments, thecircumference of such particle debris 340 is on the order of 200±50 nm.

FIG. 4B is a diagram 404 of measured contamination of the wafer clampafter stone cleaning has been performed in accordance with someembodiments. As shown in the successive panels from left to right, thesize of the debris particles 340 is decreased during application of thestone cleaning in a standard cleaning operation.

FIG. 4C is a diagram 406 of measured contamination adsorbed on thespinning cleaning electrode 360 in accordance with some embodiments,when performed after the stone cleaning. The left panel of the diagram406 shows the smaller particle debris 342 adsorbed on the surface of thecircular cleaning electrode 360, including on the first semicircle 362and the second semicircle 364. The right panel shows the adsorbedparticle debris 342 in greater detail. The symmetric bipolar electricfield 361 generated by the cleaning electrode 360 is also shown withrespect to the insulating material 363 thereon.

FIG. 5A and FIG. 5B illustrate a computer system 500 for controlling thesystem 10 and its components in accordance with various embodiments ofthe present disclosure. FIG. 5A is a schematic view of a computer system500 that controls the system 10 of FIGS. 1A-1B. In some embodiments, thecomputer system 500 is programmed to initiate a process for monitoringcontamination levels of wafer holding tools using the spectral and/orcharge monitoring system 385 which is in communication therewith, andprovide an alert that cleaning is required. In some embodiments,manufacturing of semiconductor devices is halted in response to such analarm. As shown in FIG. 5A, the computer system 500 is provided with acomputer 501 including an optical disk read only memory (e.g., CD-ROM orDVD-ROM) drive 505 and a magnetic disk drive 506, a keyboard 502, amouse 503 (or other similar input device), and a monitor 504.

FIG. 5B is a diagram showing an internal configuration of the computersystem 500. In FIG. 5B, the computer 501 is provided with, in additionto the optical disk drive 505 and the magnetic disk drive 506, one ormore processors 511, such as a micro-processor unit (MPU) or a centralprocessing unit (CPU); a read-only memory (ROM) 512 in which a programsuch as a boot up program is stored; a random access memory (RAM) 513that is connected to the processors 511 and in which a command of anapplication program is temporarily stored, and a temporary electronicstorage area is provided; a hard disk 514 in which an applicationprogram, an operating system program, and data are stored; and a datacommunication bus 515 that connects the processors 511, the ROM 512, andthe like. Note that the computer 501 may include a network card (notshown) for providing a connection to a computer network such as a localarea network (LAN), wide area network (WAN) or any other useful computernetwork for communicating data used by the computer system 500 and thesystem 10. In various embodiments, the controller 500 communicates viawireless or hardwired connection to the system 10 and its componentsthat are described herein.

The program for causing the computer system 500 to execute the processfor controlling the apparatus 10 of FIGS. 1A-1B, and components thereofand/or to execute the processes for manufacturing a semiconductor deviceaccording to the embodiments disclosed herein are stored in an opticaldisk 521 or a magnetic disk 522, which is inserted into the optical diskdrive 505 or the magnetic disk drive 506, and transmitted to the harddisk 514. Alternatively, the program is transmitted via a network (notshown) to the computer system 500 and stored in the hard disk 514. Atthe time of execution, the program is loaded into the RAM 513. Theprogram is loaded from the optical disk 521 or the magnetic disk 522, ordirectly from a network in various embodiments.

The stored programs do not necessarily have to include, for example, anoperating system (OS) or a third party program to cause the computer 501to execute the methods disclosed herein. The program may only include acommand portion to call an appropriate function (module) in a controlledmode and obtain desired results in some embodiments. In variousembodiments described herein, the controller 500 is in communicationwith the lithography system 10 to control various functions thereof.

The controller 500 is coupled to the apparatus 10 in variousembodiments. The controller 500 is configured to provide control data tothose system components and receive process and/or status data fromthose system components. For example, the controller 500 comprises amicroprocessor, a memory (e.g., volatile or non-volatile memory), and adigital I/O port capable of generating control voltages sufficient tocommunicate and activate inputs to the processing system, as well asmonitor outputs from the lithography apparatus 10. In addition, aprogram stored in the memory is utilized to control the aforementionedcomponents of the lithography apparatus 10 according to a processrecipe. Furthermore, the controller 500 is configured to analyze theprocess and/or status data, to compare the process and/or status datawith target process and/or status data, and to use the comparison tochange a process and/or control a system component. In addition, thecontroller 500 is configured to analyze the process and/or status data,to compare the process and/or status data with historical process and/orstatus data, and to use the comparison to predict, prevent, and/ordeclare a fault or alarm.

FIG. 6 is a flowchart of a cleaning process 600 using the spinningcleaning electrode 360 in accordance with some embodiments. In someembodiments, the process 600 is controlled by the controller 500. Thecleaning process 600 commences after a lithography operation by theapparatus 10 is halted and the stone cleaning described above ends(operation 602) in various embodiments. Next, at operation 604, thecleaning electrode 360 is charged in various embodiments. The motor 390is activated to spin the cleaning electrode 360 (operation 606) in oneof the clockwise or counter-clockwise directions, or in both suchdirections in an alternating fashion, in various embodiments. The waferchuck 322 is then placed in proximity to the spinning cleaning electrode360 (operation 608) in various embodiments. In some embodiments, thewafer chuck 322 is placed under the spinning cleaning electrode 360. Insome embodiments, the wafer chuck 322 is placed under the cleaningelectrode 360, and then spinning of the cleaning electrode 360commences. At operation 610, the wafer chuck 322 is moved directionallyunder electrode across its entire surface area in various embodiments.Such movement is lateral in a horizontal and/or vertical direction, aswell as circularly as described previously above in some embodiments.Next, at operation 612, it is determined whether the wafer chuck 322 hasbeen sufficiently cleaned. This determination is made by the controller500 in some embodiments. The determination is made based on the durationof the cleaning process in some embodiments. The determination is madeusing data from the spectral and/or charge monitoring system 385 withrespect to a threshold value or range in some embodiments. If thesurface of the wafer chuck 322 is not sufficiently clean, the process600 returns to operation 610 above in various embodiments. If thesurface of the wafer chuck 322 is sufficiently clean, the process 600instead continues to operation 614 where the cleaning process ends. Thecleaning electrode 360 is then cleaned using debris collector 370 or thelike using mechanical and/or electrical (i.e. reverse charge) processesin various embodiments. In various embodiments, the cleaning electrode360 is cleaned in this manner without removing it from the processingchamber 306, thereby further reducing maintenance down-time.

This disclosure introduces methods and apparatus for cleaning the smallelectrically-charged particles 342 which accumulate between the burls324 of a wafer chuck 322 using electrical force. Removal of suchparticles 342 greatly increases EUV lithographic system availability andreduces the cost of parts removal and replacement. Moreover, themanufacturing yield of the apparatus 10 is improved due to fewer overlayerrors, resulting in improved exposure quality on the scanner side.

According to various embodiments, a lithography apparatus includes awafer chuck having a surface, a cleaning electrode for generating anelectric field, and a motor for spinning the cleaning electrode. In suchembodiments, when the cleaning electrode is spinning in proximity to thewafer chuck, electrically-charged particles are adsorbed by the cleaningelectrode from the wafer chuck surface. In some embodiments, the waferclamp is an electrostatic chuck. In some embodiments, the shape of thecleaning electrode has at least two axes of symmetry. In someembodiments, the shape is circular and the electric field is a bi-polarelectric field. In some embodiments, the shape is a regular polygon andthe electric field is a bi-polar electric field. In some embodiments,the shape is one of a star shape and a fan shape, and the electric fieldis a bi-polar electric field. In some embodiments, the apparatusincludes at least one of an alternating current (AC) power source, adirect current (DC) power source and a wireless electrical power sourcefor charging the cleaning electrode. In some embodiments, the cleaningelectrode has a first positively-charged portion and secondnegatively-charged portion, where the first and second portions areelectrically isolated. In some embodiments, the first positively-chargedportion is approximately half the shape and the secondnegatively-charged portion is approximately half the shape. In someembodiments, the apparatus includes an axle connecting the motor to thecleaning electrode. In some embodiments, the apparatus includes a slipring for powering the cleaning electrode. In some embodiments, theapparatus includes a debris collector for removing theelectrically-charged particles from the cleaning electrode.

According to various embodiments, a method for removing contaminationfrom a lithography device includes disposing a bi-polar electrode abovea surface of a wafer clamp, charging the bi-polar electrode; spinningthe bi-polar electrode; and moving at least one of the wafer clamp andthe bi-polar electrode relative to each other such thatelectrically-charged particles on the surface of the wafer clamp areattracted onto a surface of the bi-polar electrode while the bi-polarelectrode is spinning. In some embodiments, the bi-polar electrodegenerates a symmetric electric field. In some embodiments, the bi-polarelectrode spins in one or both of the clockwise and a counterclockwisedirections. In some embodiments, the wafer clamp is moved laterallyand/or circularly relative to the bi-polar electrode. In someembodiments, the wafer surface is cleaned with a cleaning stone whichgenerates the electrically-charged particles that are later cleaned bythe cleaning electrode. In some embodiments, the electrically-chargedparticles are subsequently removed from the bi-polar electrode using adebris collector disposed within the lithography apparatus.

According to various embodiments, a method for removing contaminationfrom a lithography device includes removing large contaminant particleshaving a first size from a wafer chuck surface using a cleaning stone.In such embodiments, a cleaning electrode having a symmetrical surfacewith at least two axes of symmetry is then placed above the wafer chucksurface. In such embodiments, the cleaning electrode is then charged togenerate a symmetrical electric field. In such embodiments, the cleaningelectrode spins above the wafer chuck surface so thatelectrically-charged residue particles having a second size are adsorbedby the symmetrical surface of the electrode from the wafer chuck surfaceusing the symmetrical electric field, wherein the first size is largerthan the second size. In some embodiments, the wafer chuck surface ismoved laterally and/or circularly relative to the cleaning electrodewhile the cleaning electrode is spinning.

The foregoing outlines features of several embodiments or examples sothat those skilled in the art better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodiments orexamples introduced herein. Those skilled in the art should also realizethat such equivalent constructions do not depart from the spirit andscope of the present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A lithography apparatus comprising: a wafer chuckhaving a surface; a cleaning electrode configured to generate anelectric field; and a motor for spinning the cleaning electrode, whereinwhen the cleaning electrode is spinning in proximity to the wafer chuck,electrically-charged particles are adsorbed by the cleaning electrodefrom the surface.
 2. The apparatus of claim 1, wherein the wafer clampis an electrostatic chuck.
 3. The apparatus of claim 1, wherein thecleaning electrode comprises a shape with at least two axes of symmetry.4. The apparatus of claim 3, wherein the shape is circular and theelectric field is a bi-polar electric field.
 5. The apparatus of claim3, wherein the shape is a regular polygon and the electric field is abi-polar electric field.
 6. The apparatus of claim 3, wherein the shapeis one of a star shape and a fan shape, and wherein the electric fieldis a bi-polar electric field.
 7. The apparatus of claim 3 furthercomprising at least one of: an alternating current (AC) power source, adirect current (DC) power source and a wireless electrical power sourcefor charging the cleaning electrode.
 8. The apparatus of claim 7, thecleaning electrode further having a first positively-charged portion andsecond negatively-charged portion that are electrically isolated.
 9. Theapparatus of claim 8, wherein the first positively-charged portioncomprises substantially half the shape and the second negatively-chargedportion comprises substantially half the shape.
 10. The apparatus ofclaim 1, further comprising a motor for spinning the cleaning electrodeand an axle connecting the motor to the cleaning electrode.
 11. Theapparatus of claim 1, further comprising a slip ring for powering thecleaning electrode.
 12. The apparatus of claim 1, further comprising adebris collector for removing the electrically-charged particles fromthe cleaning electrode.
 13. A method for removing contamination from alithography device, comprising: disposing a bi-polar electrode above asurface of a wafer clamp; charging the bi-polar electrode; spinning thebi-polar electrode; and moving at least one of the wafer clamp and thebi-polar electrode relative to each other such that electrically-chargedparticles on the surface of the wafer clamp are attracted onto a surfaceof the bi-polar electrode while the bi-polar electrode is spinning. 14.The method of claim 13, further comprising: generating a symmetricelectric field with the bi-polar electrode.
 15. The method of claim 14,further comprising: spinning the bi-polar electrode in at least one of aclockwise and a counterclockwise direction.
 16. The method of claim 14,further comprising: moving the wafer clamp at least one of laterally andcircularly relative to the bi-polar electrode.
 17. The method of claim14, further comprising: prior to said disposing, cleaning the wafersurface with a cleaning stone.
 18. The method of claim 14, furthercomprising removing the electrically-charged particles from the bi-polarelectrode using a debris collector.
 19. A lithography apparatus forcleaning a wafer chuck, comprising: a bi-polar electrode having asymmetrical surface with at least two axes of symmetry, and comprising afirst and a second pole portions that are respectively formed at a firstand a second ends of the symmetrical surface and oppositely charged; anda motor configured to spin the bi-polar electrode so thatelectrically-charged particles are absorbed from a surface of the waferchuck when the bi-polar electrode is in proximity to the wafer chuck.20. The apparatus of claim 19, wherein the first and the second poleportions are isolated from each other by an insulting material on thebi-polar electrode, and wherein the insulating material comprises one ofglass, plastic resin, an air gap or rubber.